The long-term goal of this proposal is to investigate the molecular and cellular mechanisms that underlie the establishment of nociceptive circuits in the dorsal spinal cord. The function of circuits that sense noxious stimuli critically depends on precise neuronal connectivity between primary sensory afferents and spinal cord dorsal horn neurons. However, the molecular and cellular mechanisms that govern the developing dorsal horn circuits are poorly understood. Drg11, a paired homeodomain transcription factor, is expressed by both primary sensory neurons in dorsal root ganglia (DRG) and their central targets in the dorsal horn. In Drg11-/-mice, the spatio-temporal patterning of primary sensory afferent fiber ingrowth into the dorsal horn is disrupted. A cascade of sequelae consequent to these initial defects lead to cell death in the dorsal horn, and significantly attenuated responses to noxious stimuli in adults. Lmx1b, a lim homeodomain transcription factor and an upstream regulator of Drg11, is expressed in the dorsal horn but not in the DRG. In Lmx1b-/-mice, the entry of primary afferents in the dorsal horn is virtually blocked from the beginning. Therefore, Drg11 and Lmx1b are essential transcriptional determinants of the mechanisms that underlie the formation of pain circuits in mammals. Here we propose to study the precise role of Drg11 and Lmx1b in dorsal horn circuits by further analyzing the phenotype of Drg11-/- and Lmx1b-/- mice using both in vivo and in vitro approaches. We will also determine the roles of Drg11 and Lmx1b during postnatal dorsal horn development by generating Drg11 and Lmx1b conditional knockouts. Finally, we have initiated the search for dorsal horn-specific genes and will dissect the genetic cascade that controls the development of the dorsal spinal cord. The proposed study should provide a clearer molecular and cellular understanding of Drg11 and Lmx1b in the formation of dorsal horn nociceptive circuits. As defects in dorsal horn development could have a severe consequence on pain sensation, our work will ultimately lead to a better understanding of pain circuits and thus treatment of pain, a major clinical problem in human health.